OCaml Programming Guidelines

This is a set of reasonable guidelines for writing OCaml programs that reflect the consensus among veteran OCaml programmers.

OCaml source code can be formatted automatically with OCamlFormat, so you don't have to worry about formatting it by hand. You can speed up code review by just focusing on the important parts. Nevertheless, some best practices are not automated, so they're documented in this article. If you prefer to format your code manually, there are some formatting guidelines at the end of this article.

General Guidelines

Be Simple and Readable

The time you spend typing the programs is negligible compared to the time spent reading them. That's the reason why you save a lot of time if you work hard to optimise readability.

The time you are "wasting" to get a simpler program today will pay off a hundredfold in the future during the uncountable modifications and readings of the program (starting with the first debugging).

Writing programs law: A program is written once, modified ten times, and read 100 times. So it's beneficial to simplify its writing, always keep future modifications in mind, and never jeopardize readability.

Naming Complex Arguments

In place of

let temp =
  f x y z
    expressionother large


let t =
  expressionand u =
other large
let temp =
  f x y z t u in

Naming Anonymous Functions

In the case of an iterator whose argument is a complex function, define the function by a let binding as well. In place of

  (function x ->


let f x =
  blabla in
List.map f l

Justification: Much clearer, in particular if the name given to the function is meaningful.

Programming Guidelines

How to Program

Always put your handiwork back on the bench,
then polish it and repolish it.

Write Simple and Clear Programs

Reread, simplify, and clarify at every stage of creation. Use your head!

Subdivide Your Programs Into Little Functions

Small functions are easier to master.

Factor out snippets of repeated code by defining them in separate functions

Sharing code obtained in this way facilitates maintenance, since every correction or improvement automatically spreads throughout the program. Besides, the simple act of isolating and naming a snippet of code sometimes lets you identify an unsuspected feature.

Never copy-paste code when programming

Pasting code almost surely indicates introducing a code sharing default and neglecting to identify and write a useful auxiliary function. Hence, it means that some code sharing is lost in the program. Losing code sharing implies that you will have more problems afterwards for maintenance. A bug in the pasted code has to be corrected at each occurrence of the bug in each copy of the code!

Moreover, it is difficult to identify that the same ten lines of code is repeated twenty times throughout the program. By contrast, if an auxiliary function defines those ten lines, it is fairly easy to see and find where those lines are used: simply where the function is called. If code is copy-pasted all over the place, then the program is more difficult to understand.

In conclusion, copy-pasting code leads to programs that are more difficult to read and more difficult to maintain. It must be banished.

How to Comment Programs

Don't hesitate to comment when there's a difficulty. If there's no difficulty, there's no point in commenting. It merely creates unnecessary noise.

Avoid comments in the bodies of functions. Prefer one comment at the beginning of the function that explains how a particular algorithm works. Once more, if there is no difficulty, there is no point in commenting.

Avoid Nocuous Comments

A nocuous comment is a comment that does not add any value, e.g., trivial information. The nocuous comment is evidently not of interest; it is a nuisance that uselessly distracts the reader. It is often used to fulfill some strange criteria related to the so-called software metrology, i.e., the ratio number of comments / number of lines of code. This arbitrary ratio has no theoretical or practical interpretation.

Absolutely avoid nocuous comments.

An example of what to avoid, the following comment uses technical words and is thus masquerading as a real comment, but it has no additional information of interest:

  Function print_lambda:
  print a lambda-expression given as argument.

  Arguments: lam, any lambda-expression.
  Returns: nothing.

  Remark: print_lambda can only be used for its side effect.
let rec print_lambda lam =
  match lam with
  | Var s -> printf "%s" s
  | Abs l -> printf "\\ %a" print_lambda l
  | App (l1, l2) ->
     printf "(%a %a)" print_lambda l1 print_lambda l2

Usage in Module Interface

The function's usage must appear in the module's interface that exports it, not in the program that implements it. Choose comments as in the OCaml system's interface modules, which will subsequently automatically extract the documentation of the interface module if necessary.

Use Assertions

Use assertions as much as possible, as they let you avoid verbose comments while allowing a useful verification upon execution.

For example, the conditions to validate a function's arguments are usefully verified by assertions.

let f x =
  assert (x >= 0);

Note as well that an assertion is often preferable to a comment because it's more trustworthy. An assertion is forced to be pertinent because it is verified upon each execution, while a comment can quickly become obsolete, making it detrimental to understanding the program.

Comments line by line in imperative code

When writing difficult code, and particularly in case of highly imperative code with a lot of memory modifications (physical mutations in data structures), it is sometimes mandatory to comment inside the body of functions to explain the algorithm's implementation encoded here or to follow successive invariant modifications that the function must maintain. Once more, if there is some difficulty commenting is mandatory, for each program line if necessary.

How to Choose Identifiers

It's hard to choose identifiers whose name evokes the meaning of the corresponding portion of the program. This is why you must devote particular care to this, emphasising clarity and regularity of nomenclature.

Don't use abbreviations for global names

Global identifiers (including the names of functions) can be long because it's important to understand what purpose they serve far from their definition.

Separate words by underscores: (int_of_string, not intOfString)

Case modifications are meaningful in OCaml. In effect, capitalised words are reserved for constructors and module names. In contrast, regular variables (functions or identifiers) must start with a lowercase letter. Those rules prevent proper usage of case modification for word separation in identifiers. The first word starts the identifier, hence it must be lowercase, and it is forbidden to choose IntOfString as a function name.

Always give the same name to function arguments which have the same meaning

If necessary, make this nomenclature explicit in a comment at the top of the file. If there are several arguments with the same meaning, then attach numeral suffixes to them.

Local identifiers can be brief and should be reused from one function to another

This augments style consistency. Avoid using identifiers whose appearance can lead to confusion, such as l or O, which are easy to confuse with 1 and 0.


let add_expression expr1 expr2 = ...
let print_expression expr = ...

A tolerated exception to the recommendation not to use capitalisation to separate words within identifiers when interfacing with existing libraries which use this naming convention. This lets OCaml library users to orient themselves in the original library documentation more easily.

How to Use Modules

Subdividing into modules

You must subdivide your programs into coherent modules.

For each module, you must explicitly write an interface.

For each interface, you must document the things defined by the module: functions, types, exceptions, etc.

Opening modules

Avoid open directives, using instead the qualified identifier notation. Thus you will prefer short but meaningful module names.

Justification: The use of unqualified identifiers is ambiguous and gives rise to difficult-to-detect semantic errors.

let lim = String.length name - 1 in
let lim = Array.length v - 1 in
... List.map succ ...
... Array.map succ ...

When to use open modules rather than leaving them closed

Consider it normal to open a module that modifies the environment and brings other versions of an important set of functions. For example, the Format module automatically provides indented printing. This module redefines the usual printing functions print_string, print_int, print_float, etc., so when you use Format, open it systematically at the top of the file.
If you don't open Format, you could miss a printing function qualification, and this could be perfectly silent, since many of Format's functions have a counterpart in the default environment (Stdlib). Mixing printing functions from Format and Stdlib leads to subtle bugs in the display that are difficult to trace. For instance:

let f () =
  Format.print_string "Hello World!"; print_newline ()

is bogus since it does not call Format.print_newline to flush the pretty-printer queue and output "Hello World!". Instead "Hello World!" is stuck into the pretty-printer queue, while Stdlib.print_newline outputs a carriage return on the standard output.

If Format is printing on a file and standard output is the terminal, the user will have a difficult time finding that the file is missing a carriage return (and the display of material on the file is strange, since boxes that should be closed by Format.print_newline are still open), while a spurious carriage return appeared on the screen!

For the same reason, open large libraries such as the one with arbitrary-precision integers so as not to burden the program that uses them.

open Num

let rec fib n =
  if n <= 2 then Int 1 else fib (n - 1) +/ fib (n - 2)

Justification: The program would be less readable if you had to qualify all the identifiers.

In a program where type definitions are shared, it's beneficial to gather these definitions into one or more module(s) without implementations (containing only types). Then it's acceptable to systematically open the module that exports the shared type definitions.


Never be afraid of overusing pattern-matching! On the other hand, be careful to avoid nonexhaustive pattern-matching constructs. Complete them with care, without using a “catch-all” clause such as | _ -> ... or | x -> ... when it's unnecessary (for example when matching a concrete type defined within the program). See also the next section: compiler warnings.

Compiler Warnings

Compiler warnings are meant to prevent potential errors, which is why you absolutely must heed them and correct your programs if compiling them produces such warnings. Besides, programs whose compilation produces warnings have an odor of amateurism which certainly doesn't suit your own work!

Pattern-matching warnings

Warnings about pattern-matching must be treated with the upmost care.

  • Those with useless clauses should be eliminated, of course.
  • For nonexhaustive pattern-matching, you must complete the corresponding pattern-matching construct without adding a default case “catch-all”, such as | _ -> ... , but rather with an explicit constructor list not examined by the rest of the construct, e.g., | Cn _ | Cn1 _ -> ... .

Justification: It's not really more complicated to write it this way, and this allows the program to evolve more safely. In effect, the addition of a new constructor to the datatype matched will produce an alert anew, which will allow the programmer to add a clause corresponding to the new constructor, if warranted. On the contrary, the “catch-all” clause will make the function compile silently, and it might be thought that the function is correct, as the new constructor will be handled by the default case.

  • Nonexhaustive pattern-matches induced by clauses with guards must also be corrected. A typical case consists in suppressing a redundant guard.

Destructuring let bindings

A “destructuring let binding” is one which binds several names to several expressions simultaneously. You pack all the names you want bound into a collection such as a tuple or a list, then you correspondingly pack all the expressions into a collective expression. When the let binding is evaluated, it unpacks the collections on both sides and binds each expression to its corresponding name. For example, let x, y = 1, 2 is a destructuring let binding that performs both the bindings let x = 1 and let y = 2 simultaneously.

The let binding is not limited to simple identifier definitions. You can use it with more complex or simpler patterns. For instance:

  • let with complex patterns:
    let [x; y] as l = ...
    simultaneously defines a list l and its two elements x and y.
  • let with simple pattern:
    let _ = ... does not define anything, it just evaluate the expression on the right hand side of the = symbol.

The destructuring let must be exhaustive

Only use destructuring let bindings when the pattern-matching is exhaustive (the pattern can never fail to match). Typically, you will thus be limited to product-type definitions (tuples or records) or, with a single case, variant-type definitions. At any other time, use an explicit match ... with construct.

  • let ... in: destructuring let that gives a warning must be replaced by an explicit pattern-matching. For instance, instead of let [x; y] as l = List.map succ (l1 @ l2) in expression write:
match List.map succ (l1 @ l2) with
| [x; y] as l -> expression
| _ -> assert false
  • Global definition with destructuring let statements should be rewritten with explicit pattern-matching and tuples:
let x, y, l =
  match List.map succ (l1 @ l2) with
  | [x; y] as l -> x, y, l
  | _ -> assert false

Justification: There is no way to make the pattern-matching exhaustive if you use general destructuring let bindings.

Sequence warnings and let _ = ...

When the compiler emits a warning about a sequential expression type, you must explicitly indicate that you want to ignore this expression's result. To this end:

  • use a vacuous binding and suppress the sequence warning of
List.map f l;
print_newline ()


let _ = List.map f l in
print_newline ()
  • You can also use the predefined function ignore : 'a -> unit, which ignores its argument to return unit.
ignore (List.map f l);
print_newline ()
  • Regardless, the best way to suppress this warning is to understand why the compiler emits it. The compiler warns you because your code computes a result that is useless since it's just deleted after computation. Hence, if useful at all, this computation is performed only for its side effects; hence, it should return unit.

Most of the time, the warning indicates the use of the wrong function, a probable confusion between the side-effect-only version of a function (which is a procedure whose result is irrelevant) with its functional counterpart (whose result is meaningful).

In the example mentioned above, the first situation prevailed, and the programmer should have called iter instead of map, then simply write:

List.iter f l;
print_newline ()

In actual programs, the suitable (side-effect-only) function may not exist and must be written. Frequently, a careful separation of the procedural part from the functional part of the function elegantly solves the problem, and the resulting program just looks better afterwards! For instance, you would turn the problematic definition

let add x y =
  if x > 1 then print_int x;
  print_newline ();
  x + y;;

into the clearer, separate definition and change old calls to add accordingly.

In any case, use the let _ = ... construction exactly in those cases where you want to ignore a result. Don't systematically replace sequences with this construction.

Justification: Sequences are much clearer! Compare e1; e2; e3 to

let _ = e1 in
let _ = e2 in

The hd and tl Functions

Don't use the hd and tl functions, but rather pattern-match the list argument explicitly.

Justification: This is just as brief as and much clearer than using hd and tl, which must be protected by try... with... to catch the exception that might be raised by these functions.


for loops

To simply traverse an array or a string, use a for loop.

for i = 0 to Array.length v - 1 do

If the loop is complex or returns a result, use a recursive function.

let find_index e v =
  let rec loop i =
    if i >= Array.length v then raise Not_found else
    if v.(i) = e then i else loop (i + 1) in
  loop 0;;

Justification: The recursive function lets you code any loop whatsoever simply, even a complex one, e.g., with multiple exit points or with strange index steps (steps depending on a data value for example).

Besides, the recursive loop avoids the use of mutables whose value can be modified inside any part of the loop whatsoever (or even outside). On the contrary, the recursive loop explicitly takes the values susceptible to change during the recursive calls as arguments.

while loops

While loops law: Beware! A while loop is usually wrong, unless its loop invariant has been explicitly written.

The main use of the while loop is the infinite loop while true do .... You get out of it through an exception, generally on the program's termination.

Other while loops are hard to use, unless they come from canned programs from algorithm courses where they were proved.

Justification: while loops require one or more mutables, so the loop condition changes value and the loop finally terminates. To prove their correctness, you must discover the loop invariants, an interesting but difficult sport.


Don't be afraid to define your own exceptions in your programs, but on the other hand, use as many exceptions predefined by the system as possible. For example, every search function that fails should raise the predefined exception Not_found. Be careful to handle the exceptions that a function call might raise with the help of a try ... with.

Handling all exceptions by try ... with _ -> is usually reserved for the program's main function. If you must catch every exception in order to maintain an algorithm's invariant, be careful to name the exception and re-raise it after having reset the invariant. Typically:

let ic = open_in ...
and oc = open_out ... in
  treatment ic oc;
  close_in ic; close_out oc
with x -> close_in ic; close_out oc; raise x

Justification: try ... with _ -> silently catches all exceptions, even those which have nothing to do with the computation at hand (for example, an interruption will be captured and the computation will continue anyway!).

Data Structures

One of the great strengths of OCaml is the power of definable data structures and the simplicity of manipulating them. So you must take advantage of this to the fullest extent! Don't hesitate to define your own data structures. In particular, don't systematically represent enumerations by whole numbers, nor enumerations with two cases by Booleans. Examples:

type figure =
   | Triangle | Square | Circle | Parallelogram
type convexity =
   | Convex | Concave | Other
type type_of_definition =
   | Recursive | Non_recursive

Justification: A Boolean value often prevents intuitive understanding of the corresponding code. For example, if type_of_definition is coded by a Boolean, what does true signify? A “normal” definition (that is, non-recursive) or a recursive definition?

In the case of an enumerated type encode by an integer, it is very difficult to limit the range of acceptable integers. One must define construction functions that will ensure the program's mandatory invariants (and afterwards verify no values have been built directly) or add assertions in the program and guards in pattern-matchings. This is not good practice when the definition of a sum type elegantly solves this problem along with the additional benefit of firing pattern-matching's full power and the compiler's verifications of exhaustiveness.

Criticism: For binary enumerations, one can systematically define predicates whose names carry the semantics of the Boolean that implements the type. For instance, we can adopt the convention that a predicate ends by the letter p. Then, in place of defining a new sum type for type_of_definition, we will use a predicate function recursivep that returns true if the definition is recursive.

Answer: This method is specific to binary enumeration and cannot be easily extended; moreover, it is not well suited to pattern-matching. For instance, a typical pattern-matching for definitions encoded by | Let of bool * string * expression would look like:

| Let (_, v, e) as def ->
   if recursivep def then code_for_recursive_case
   else code_for_non_recursive_case

or, if recursivep, can be applied to booleans:

| Let (b, v, e) ->
   if recursivep b then code_for_recursive_case
   else code_for_non_recursive_case

Contrast this with an explicit encoding:

| Let (Recursive, v, e) -> code_for_recursive_case
| Let (Non_recursive, v, e) -> code_for_non_recursive_case

The difference between the two programs is subtle, and you may think that it's just a matter of taste; however, the explicit encoding is definitively more robust to modifications and fits better with the language.

A contrario, it is not necessary to systematically define new types for Boolean flags when the interpretation of constructors true and false are clear. The usefulness of the following types' definitions are then questionable:

type switch = On | Off
type bit = One | Zero

The same objection is admissible for enumerated types represented as integers when those integers have an evident interpretation with respect to the represented data.

When to Use Mutables

Mutable values are useful and sometimes indispensable for simple and clear programming. Nevertheless, you must use them with discernment because OCaml's normal data structures are immutable. They are preferred for the clarity and safety of programming they allow.


OCaml's iterators are a powerful and useful feature. However, you should not overuse them nor neglect them. They are provided by libraries and have every chance of being correct and well thought out by the library's author, so it's useless to reinvent them.


let square_elements elements = List.map square elements

rather than:

let rec square_elements = function
  | [] -> []
  | elem :: elements -> square elem :: square_elements elements

On the other hand, avoid writing:

let iterator f x l =
  List.fold_right (List.fold_left f) [List.map x l] l

even though you get:

  let iterator f x l =
    List.fold_right (List.fold_left f) [List.map x l] l;;
  iterator (fun l x -> x :: l) (fun l -> List.rev l) [[1; 2; 3]]

In case of express need, be sure to add an explanatory comment. In my opinion, it's absolutely necessary!

How to Optimize Programs

Pseudo law of optimisation: No optimisation a priori.
No optimisation a posteriori either.

Above all, program simply and clearly. Don't start optimising until the program's bottleneck has been identified (in general, after a few routines). Then optimisation consists of changing the algorithm's complexity above all. This often happens through redefining the data structures manipulated and completely rewriting the part of the program that poses a problem.

Justification: Clarity and correctness of programs take precedence. Besides, in a substantial program, it is practically impossible to identify a priori the parts of the program whose efficiency is of prime importance.

How to Choose Between Classes and Modules

Use OCaml classes when you need inheritance, i.e., incremental refinement of data and their functionality.

Use conventional data structures (in particular, variant types) when you need pattern-matching.

Use modules when the data structures are fixed and their functionality is equally fixed or it's enough to add new functions in the programs which use them.

Clarity of OCaml Code

The OCaml language includes powerful constructs that allow simple and clear programming. The main problem to obtain crystal clear programs is to use them appropriately.

The language features numerous programming styles (or programming paradigms): imperative programming (based on the notion of state and assignment), functional programming (based on the notion of function, function results, and calculus), object oriented programming (based on the notion of objects encapsulating a state and some procedures or methods that can modify the state). The first work of the programmer is to choose the programming paradigm that best fits the problem at hand. When using a programming paradigm, the difficulty is to use the language construct that expresses the computation to implement the algorithm in the most natural and easiest way.

Style dangers

Concerning programming styles, one can usually observe the two symmetrical problematic behaviors. On one hand, the “all imperative” way (systematic usage of loops and assignment), and on the other hand, the “purely functional” way (never use loops nor assignments). The “100% object” style will certainly appear in the future.

  • The “too much imperative” danger:
    • It is a bad idea to use imperative style to code a function that is naturally recursive. For instance, to compute the length of a list, you shouldn't write:
let list_length l =
  let l = ref l in
  let res = ref 0 in
  while !l <> [] do
    incr res; l := List.tl !l

in place of the following recursive function that's so simple and clear:

let rec list_length = function
  | [] -> 0
  | _ :: l -> 1 + list_length l

(For those that would contest the equivalence of those two versions, see the note below).

  • Another common “over-imperative error” in the imperative world is not to systematically choose the simple for loop to iterate on a vector's element, but instead to use a complex while loop with one or two references. Too many useless assignments means too many opportunity for errors.

  • This category of programmer feels that the mutable keyword in the record-type definitions should be implicit.

  • The “too much functional” danger:

    • The programmer that adheres to this dogma avoids using arrays and assignment. In the most severe cases, one observes a complete denial of writing any imperative construction, even when it's evidently the most elegant way to solve the problem.
    • Characteristic symptoms: systematic rewriting for loops with recursive functions, using lists in contexts where imperative data structures seem to be mandatory to anyone, passing numerous global parameters of the problem to every function, even when a global reference perfectly avoid these spurious parameters that are mainly invariants that must be repeatedly passed.
    • This programmer feels that the mutable keyword in record-type definitions should be suppressed from the language.

OCaml code generally considered unreadable

The OCaml language includes powerful constructs which allow simple and clear programming. However, the power of these constructs also lets you write uselessly complicated code to the point where you get a perfectly unreadable program.

Here are a number of common ways to write overly-complicated code:

  • Use useless (hence novice for readability) if then else, as in
let flush_ps () =
  if not !psused then psused := true

or (more subtle)

let sync b =
  if !last_is_dvi <> b then last_is_dvi := b
  • Code one construct with another. For example, code a let ... in by the application of an anonymous function to an argument. You would write
(fun x y -> x + y)
   e1 e2

instead of simply writing

let x = e1
and y = e2 in
x + y
  • Systematically code sequences with let in bindings.

  • Mix computations and side effects, particularly in function calls. Recall that the evaluation order of function call arguments is unspecified, which implies that you must not mix side effects and computations in function calls. However, when there is only one argument, you might take advantage of this to perform a side effect within the argument, although this is extremely troublesome for the reader, albeit without endangering the program semantics. This should be absolutely forbidden.

  • Misuse of iterators and higher-order functions (i.e., over- or underuse). For example, it's better to use List.map or List.iter than to write their equivalents inline by using your own recursive functions. Even worse, don't use List.map or List.iter, but rather write their equivalents in terms of List.fold_right and List.fold_left.

  • Another efficient way to write unreadable code is to mix all or some of these methods. For example:

(fun u -> print_string "world"; print_string u)
  (let temp = print_string "Hello"; "!" in
   ((fun x -> print_string x; flush stdout) " ";

If you naturally write the program print_string "Hello world!" in this way, please submit your work to the Obfuscated OCaml Contest.

Managing Program Development

Below are tips from veteran OCaml programmers that have served in developing the compilers. These are good examples of large, complex programs developed by small teams.

How to Edit Programs

Many developers nurture a kind of veneration towards writing their programs in the Emacs editor (GNU Emacs, in general). The editor interfaces with the language well because it is capable of syntax-coloring OCaml source code (rendering different categories of words in color, thus coloring keywords, for example).

The following two commands are considered indispensable:

  • CTRL-C-CTRL-C or Meta-X compile: launches recompilation from within the editor (using the make command).
  • CTRL-X-`: puts the cursor in the file and at the exact place where the OCaml compiler has signaled an error.

Developers describe how to use these features: the CTRL-C-CTRL-C combination recompiles the whole application; then, in case of errors, a succession of CTRL-X-` commands permits correction of all signalled errors; finally, the cycle begins again with a new recompilation launched by CTRL-C-CTRL-C.

Other Emacs tricks

The ESC-/ command (dynamic-abbrev-expand) automatically completes the word in front of the cursor with one of the words in a file being edited. This lets you always choose meaningful identifiers without the tedium of having to type extended names in your programs, i.e., the ESC-/ easily completes the identifier after typing the first letters. In case it brings up the wrong completion, each subsequent ESC-/ proposes an alternate completion.

Under Unix, the CTRL-C-CTRL-C or Meta-X compile combination, followed by CTRL-X-`, is also used to find all occurrences of a certain string in a OCaml program. Instead of launching make to recompile, launch the grep command. Then all the “error messages” from grep are compatible with the CTRL-X-` usage, which automatically takes you to the file and the place where the string is found.

How to Edit With the Interactive System

Under Unix: use the line editor ledit, which offers great editing capabilities “à la Emacs” (including ESC-/!) as well as a history mechanism that lets you retrieve previously-typed commands and even retrieve commands from one session to another. ledit is written in OCaml and can be freely downnloaded here.

How to Compile

The make utility is indispensable for managing the compilation and recompilation of programs. Sample make files can be found on The Hump. You can also consult the Makefiles for the OCaml compilers.

How to Develop as a Team: Version Control

Users of the Git software version control system never run out of good things to say about the productivity gains it brings. This system supports managing development by a programming team while imposing consistency among them, and it maintains a log of changes made to the software.
Git also supports simultaneous development by several teams, possibly dispersed among several sites linked on the Internet.

An anonymous Git read-only mirror contains the working sources of the OCaml compilers, and the sources of other software related to OCaml.

Formatting Guidelines

If you choose not to format your source code automatically with OCamlFormat, please consider these style guidelines when doing it manually.

Pseudo spaces law: never hesitate to separate words in your programs with spaces. The space bar is the easiest key to find on the keyboard, so press it as often as necessary!


A space should always follow a delimiter symbol, and spaces should surround operator symbols. It has been a great step forward in typography to separate words by spaces in order to make written texts easier to read. Do the same in your programs if you want them to be readable.

How to Write Pairs

A tuple is parenthesised, and the commas therein (delimiters) are each followed by a space: (1, 2), let triplet = (x, y, z)...

Commonly accepted exceptions:

  • Definition of the components of a pair: In place of let (x, y) = ..., you can write let x, y = ....

    Justification: The point is to define several values simultaneously, not to construct a tuple. Moreover, the pattern is set off nicely between let and =.

  • Matching several values simultaneously: It's okay to omit parentheses around n-tuples when matching several values simultaneously.

    match x, y with
    | 1, _ -> ...
    | x, 1 -> ...
    | x, y -> ...

    Justification: The point is to match several values in parallel, not to construct a tuple. Moreover, the expressions being matched are set off by match and with, while the patterns are set off nicely by | and ->.

How to Write Lists

Write x :: l with spaces around the :: (since :: is an infix operator, hence surrounded by spaces) and [1; 2; 3] (since ; is a delimiter, hence followed by a space).

How to Write Operator Symbols

Be careful to keep operator symbols separated by spaces. Not only will your formulas be more readable, but you will also avoid confusion with multicharacter operators. (Obvious exceptions to this rule are the symbols ! and ., which are not separated from their arguments.)
Example: write x + 1 or x + !y.

Justification: If you left out the spaces then x+1 would be understood, but x+!y would change its meaning, since +! would be interpreted as a multicharacter operator.

Criticism: The absence of spaces around an operator improves the readability of formulas when used to reflect the relative precedences of operators. For example x*y + 2*z makes it very obvious that multiplication takes precedence over addition.

Response: This is a bad idea, a chimera, because nothing in the language ensures that the spaces properly reflect the formula's meaning. For example x * z-1 means (x * z) - 1 and not x * (z - 1), as the proposed interpretation of spaces would seem to suggest. Besides, the problem of multicharacter symbols would keep you from using this convention in a uniform way, i.e., you couldn't leave out the spaces around the multiplication to write x*!y + 2*!z. Finally, playing with spaces is a subtle and flimsy convention, a subliminal message which is difficult to grasp when reading. If you want to make the precedences obvious, use the expressive means brought to you by the language: write parentheses.

Additional justification: Systematically surrounding operators with spaces simplifies the treatment of infix operators, which are not a complex particular case. In effect, whereas you can write (+) without spaces, you evidently cannot write (*) since (* is read as the beginning of a comment. You must write at least one space as in “( *)”, although an extra space after * is definitively preferable if you want to avoid that *) could be read, in some contexts, as the end of a comment.

All these difficulties are easily avoided if you adopt the simple rule proposed here: keep operator symbols separated by spaces.
In fact, you will quickly find that this rule isn't difficult to follow. The space bar is the greatest and best-situated key on the keyboard. It is the easiest to use because you cannot miss it!

How to Write Long Character Strings

Indent long character strings with the convention in force at that line, plus an indication of string continuation at the end of each line (a \ character at the end of the line omits white spaces at the beginning of the next line):

let universal_declaration =
  "-1- Programs are born and remain free and equal under the law;\n\
   distinctions can only be based on the common good." in

When to Use Parentheses Within an Expression

Parentheses are meaningful. They indicate the necessity of using an unusual precedence, so they should be used wisely and not sprinkled randomly throughout programs. To this end, you should know the usual precedences, i.e., the combinations of operations which do not require parentheses. Quite fortunately, this is not complicated if you know a little mathematics or strive to follow the following rules:

Arithmetic operators: the same rules as in mathematics

For example: 1 + 2 * x means 1 + (2 * x).

Function application: the same rules as those in mathematics for usage of trigonometric functions

In mathematics you write sin x to mean sin (x). In the same way sin x + cos x means (sin x) + (cos x) not sin (x + (cos x)). Use the same conventions in OCaml: write f x + g x to mean (f x) + (g x).
This convention generalises to all (infix) operators: f x :: g x means (f x) :: (g x), f x @ g x means (f x) @ (g x), and failwith s ^ s' means (failwith s) ^ s', not failwith (s ^ s').

Comparisons and Boolean operators

Comparisons are infix operators, so the preceding rules apply. This is why f x < g x means (f x) < (g x). For type reasons (and no other sensible interpretation), the expression f x < x + 2 means (f x) < (x + 2). Similarly, f x < x + 2 && x > 3 means ((f x) < (x + 2)) && (x > 3).

The relative precedences of the Boolean operators are those of mathematics

Although mathematicians have a tendency to overuse parentheses, the Boolean “or” operator is analogous to addition and the “and” to multiplication. So, just as 1 + 2 * x means 1 + (2 * x), true || false && x means true || (false && x).

How to Delimit Constructs in Programs

When it is necessary to delimit syntactic constructs in programs, use the keywords begin and end as delimiters rather than parentheses. However, using parentheses is acceptable if you do it in a consistent, systematic way.

This explicit delimiting of constructs essentially concerns pattern-matching constructs or sequences embedded within if then else constructs.

match construct in a match construct

When a match ... with or try ... with construct appears in a pattern-matching clause, it is absolutely necessary to delimit this embedded construct (otherwise subsequent clauses of the enclosing pattern-matching construct will automatically be associated with the enclosed pattern-matching construct). For example:

match x with
| 1 ->
  begin match y with
  | ...
| 2 ->

Sequences inside branches of if

In the same way, a sequence which appears in the then or else part of a conditional must be delimited:

if cond then begin
end else begin

Indentation of Programs

Landin's pseudo law: Treat your program's indentation as if it determines the meaning of your programs.

I would add to this law: be careful with the indentation in programs because, in some cases, it really defines the meaning of the program!

A program's indentation is an art which elicits many strong opinions. Here, several indentation styles are given that are drawn from experience and which have not been severely criticised.

When a justification for the adopted style has seemed obvious to me, I have indicated it. On the other hand, criticisms are also noted.

So each time, you must choose between the different styles suggested.
The only absolute rule is the first below.

Consistency of Indentation

Choose a generally accepted style of indentation, then use it systematically throughout the whole application.

Width of the Page

The page is 80 columns wide.

Justification: This width makes it possible to read the code on all displays and to print it in a legible font on a standard sheet.

Height of the Page

A function should always fit within one screenful (of about 70 lines), or in exceptional cases two, at the very most three. To go beyond this is unreasonable.

Justification: When a function goes beyond one screenful, it's time to divide it into subproblems and handle them independently. Beyond a screenful, one gets lost in the code. The indentation is not readable and is difficult to keep correct.

How Much to Indent

The change in indentation between successive lines of the program is generally 1 or 2 spaces. Pick an amount to indent and stick with it throughout the program.

Using Tab Stops

Using the tab character (ASCII character 9) is absolutely not recommended.

Justification: Between one display and another, the indentation of the program changes completely. It can also become completely wrong if the programmer used both tabulations and spaces to indent the program.

Criticism: The purpose of using tabulations is just to allow readers to indent more or less by changing the tab stops. The overall indentation remains correct, and the reader is glad to easily customise the indentation amount.

Answer: It seems almost impossible to use this method since you should always use tabulations to indent, which is hard and unnatural.

How to Indent Operations

When an operator takes complex arguments, or in the presence of multiple calls to the same operator, start the next the line with the operator, and don't indent the rest of the operation. For example:

x + y + z
+ t + u

Justification: When the operator starts the line, it is clear that the operation continues on this line.

When dealing with a “large expression” in such an operation sequence, it's preferable to define the “large expression” with the help of a let in construction than to indent the line. In place of

x + y + z


let t =
x + y + z + t

You most certainly must bind expressions too large to be written in one operation when using a combination of operators. In place of the unreadable expression

(x + y + z * t)
/ (large


let u =
(x + y + z * t) / u

These guidelines extend to all operators. For example:

let u =
x :: y
:: z + 1 :: t :: u

How to Indent Global let ... ;; Definitions

The body of a function defined globally in a module is generally indented normally. However, it's okay to treat this case specially to better offset the definition.

With a regular indentation of 1 or 2 spaces:

let f x = function
  | C ->
  | D ->

let g x =
  let tmp =
    match x with
    | C -> 1
    | x -> 0 in
  tmp + 1

Justification: No exception to the amount of indentation.

Other conventions are acceptable, for example:

  • The body is left-justified when pattern-matching.
let f x = function
| C ->
| D ->

Justification: The vertical bars separating the patterns stop when the definition is done, so it's still easy to pass on to the following definition.

Criticism: An unpleasant exception to the normal indentation.

  • The body is justified just under the name of the defined function.
let f x =
    let tmp = ... in
    try g x with
    | Not_found ->

Justification: The first line of the definition is offset nicely, so it's easier to pass from definition to definition.

Criticism: You run into the right margin too quickly.

How to Indent let ... in Constructs

The expression following a definition introduced by let is indented to the same level as the keyword let, and the keyword in, which introduces it, is written at the end of the line:

let expr1 = ... in
expr1 + expr1

In the case of a series of let definitions, the preceding rule implies. These definitions should be placed at the same indentation level:

let expr1 = ... in
let n = ... in

Justification: It is suggested that a series of let ... in constructs is analogous to a set of assumptions in a mathematical text, whence the same indentation level for all the assumptions.

Variation: some write the keyword in alone on one line to set apart the final expression of the computation:

let e1 = ... in
let e2 = ... in
let new_expr =
  let e1' = derive_expression e1
  and e2' = derive_expression e2 in
  Add_expression e1' e2'
Mult_expression (new_expr, new_expr)

Criticism: Lack of consistency.

How to Indent if ... then ... else ...

Multiple branches

Write conditions with multiple branches at the same level of indentation:

if cond1 ...
if cond2 ...
if cond3 ...

Justification: Analogous treatment to pattern-matching clauses, all aligned to the same tab stop.

If the condition's size and the expressions allow, write:

if cond1 then e1 else
if cond2 then e2 else
if cond3 then e3 else

If expressions in the branches of multiple conditions have to be enclosed (when they include statements for instance), write:

if cond then begin
  end else
if cond2 then begin
  end else
if cond3 then ...

Some suggest another method for multiple conditionals: starting each line by the keyword else:

if cond1 ...
else if cond2 ...
else if cond3 ...

Justification: elsif is a keyword in many languages, so use indentation and else if to bring it to mind. Moreover, you do not have to look at the end of line to know whether the condition is continued or another test is performed.

Criticism: Lack of consistency in the treatment of all conditions. Why use a special case for the first condition?

Yet again, choose your style and use it systematically.

Single branches

Several styles are possible for single branches, according to the size of the expressions in question and especially the presence of begin, end, or ( ) delimiters for these expressions.

When delimiting a conditional's branches, several styles are used:

( at end of line:

if cond then (
) else (

Or alternatively first begin at beginning of line:

if cond then
  end else begin

In fact, the conditional's indentation depends on their expressions' size.

If cond, e1, and e2 are small, simply write them on one line:

if cond then e1 else e2

If the expressions making up a conditional are purely functional (without side effects), we advocate binding them within the conditional by using let e = ... in when they're too big to fit on a single line.

Justification: This way you get back the simple indentation on one line, which is the most readable. As a side benefit, naming acts as an aid to comprehension.

So now we consider the case in which the expressions in question do have side effects, which keeps us from simply binding them with a let e = ... in.

If e1 and cond are small, but e2 large:

if cond then e1 else

If e1 and cond are large, but e2 small:

if cond then
else e2

If all the expressions are large:

if cond then

If there are ( ) delimiters:

if cond then (
) else (

A mixture where e1 requires ( ), but e2 is small:

if cond then (
) else e2

How to Indent Pattern-Matching Constructs

General principles

All the pattern-matching clauses are introduced by a vertical bar, including the first one.

Criticism: The first vertical bar is not mandatory. Hence, there is no need to write it.

Answer to criticism: If you omit the first bar, the indentation seems unnatural. The first case gets an indentation larger than a normal new line would necessitate. It is thus a useless exception to the correct indentation rule. It also insists on not using the same syntax for the whole set of clauses, writing the first clause as an exception with a slightly different syntax. Last, aesthetic value is doubtful (some people would say “awful” instead of “doubtful”).

Align all the pattern-matching clauses with the vertical bar that begins each clause, including the first one.

If an expression in a clause is too large to fit on one line, you must break the line immediately after the arrow of the corresponding clause. Then indent normally, starting from the beginning of the clause's pattern.

Arrows of pattern-matching clauses should not be aligned.

match or try

For a match or a try, align the clauses with the beginning of the construct:

match lam with
| Abs (x, body) -> 1 + size_lambda body
| App (lam1, lam2) -> size_lambda lam1 + size_lambda lam2
| Var v -> 1

try f x with
| Not_found -> ...
| Failure "not yet implemented" -> ...

Put the keyword with at the end of the line. If the preceding expression extends beyond one line, put with on its own line:

  let y = f x in
  if ...
| Not_found -> ...
| Failure "not yet implemented" -> ...

Justification: The keyword with on its own line shows that the program enters the pattern-matching part of the construct.

Indenting expressions inside clauses

If the expression on the right of the pattern-matching arrow is too large, cut the line after the arrow.

match lam with
| Abs (x, body) ->
   1 + size_lambda body
| App (lam1, lam2) ->
   size_lambda lam1 + size_lambda lam2
| Var v ->

Some programmers generalise this rule to all clauses as soon as one expressions overflows. They will then indent the last clause like this:

| Var v ->

Other programmers go one step further and apply this rule systematically to any clause of any pattern-matching.

let rec fib = function
  | 0 ->
  | 1 ->
  | n ->
     fib (n - 1) + fib ( n - 2)

Criticism: May be not compact enough. For simple pattern-matchings (or simple clauses in complex matchings), the rule does not benefit readability.

Justification: I don't see any reason for this rule, unless you are paid proportionally to the number of lines of code. In this case, use this rule to get more money without adding more bugs in your OCaml programs!

Pattern-matching in anonymous functions

Similarly to match or try, pattern-matching of anonymous functions, starting with function, are indented with respect to the function keyword:

   | Abs (x, body) -> 1 + size_lambda 0 body
   | App (lam1, lam2) -> size_lambda (size_lambda 0 lam1) lam2
   | Var v -> 1)

Pattern-matching in named functions

Pattern-matching in functions defined by let or let rec gives rise to several reasonable styles that obey the preceding pattern-matching rules (the one for anonymous functions being evidently excepted). See above for recommended styles.

let rec size_lambda accu = function
  | Abs (x, body) -> size_lambda (succ accu) body
  | App (lam1, lam2) -> size_lambda (size_lambda accu lam1) lam2
  | Var v -> succ accu

let rec size_lambda accu = function
| Abs (x, body) -> size_lambda (succ accu) body
| App (lam1, lam2) -> size_lambda (size_lambda accu lam1) lam2
| Var v -> succ accu

Bad Indentation of Pattern-Catching constructs

No beastly indentation of functions and case analyses.

This consists in indenting normally under the keyword match or function that has previously been pushed to the right. Don't write:

let rec f x = function
              | [] -> ...

but choose to indent the line under the let keyword:

let rec f x = function
  | [] -> ...

Justification: You bump into the margin. The aesthetic value is doubtful.

No beastly alignment of the -> symbols in pattern-matching clauses.

Careful alignment of pattern-matching arrows is considered bad practice, as exemplified in the following fragment:

let f = function
  | C1          -> 1
  | Long_name _ -> 2
  | _           -> 3

Justification: This makes it harder to maintain the program (the addition of a supplementary case can lead to changes in all indentations, so we often give up alignment at that time. In this case, it's better not to align the arrows in the first place!).

How to Indent Function Calls

Indentation to the function's name:

No problem arises except for functions with many arguments—or very complicated arguments—which can't fit on the same line. You must indent the expressions with respect to the fucntion's name (1 or 2 spaces according to the chosen convention). Write small arguments on the same line, and change lines at the start of an argument.

As far as possible, avoid arguments which consist of complex expressions. In these cases, define the “large” argument by a let construction.

Justification: No indentation problem. If the name given to the expressions is meaningful, the code is more readable.

Additional justification: If the argument's evaluation produces side effects, the let binding is in fact necessary to explicitly define the evaluation order.


Imperative and Functional Versions of list_length

The two versions of list_length are not completely equivalent in terms of complexity. The imperative version uses a constant amount of stack room to execute, whereas the functional version needs to store return addresses of suspended recursive calls (whose maximum number is equal to the length of the list argument). If you want to retrieve a constant space requirement to run the functional program, you simply must write a function that is recursive in its tail (or tail-rec). This is a function that just ends by a recursive call (which is not the case here, since a call to + has to be performed after the recursive call has returned). Just use an accumulator for intermediate results, as in:

let list_length l =
  let rec loop accu = function
    | [] -> accu
    | _ :: l -> loop (accu + 1) l in
  loop 0 l

This way, you get a program that has the same computational properties as the imperative program with the additional clarity and natural look of an algorithm that performs pattern-matching and recursive calls to handle an argument that belongs to a recursive sum data type.


Original translation from French: Ruchira Datta.

Thanks to all those who have already participated in the critique of this page: Daniel de Rauglaudre, Luc Maranget, Jacques Garrigue, Damien Doligez, Xavier Leroy, Bruno Verlyck, Bruno Petazzoni, Francois Maltey, Basile Starynkevitch, Toby Moth, Pierre Lescanne.

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